Current amplifier

ABSTRACT

A first transistor, a second transistor, a third transistor, and a fourth transistor, their source terminals being grounded, are provided. Further, a first feedback circuit connected between a gate terminal and a drain terminal in the first transistor, and having first impedance, a second feedback circuit connected between a gate terminal and a drain terminal in the second transistor, and having the first impedance, a current source for outputting a current, a first load circuit connected between the drain terminal of the first transistor and a first output terminal of the current source, and having second impedance, and a second load circuit connected between the drain terminal of the second transistor and a second output terminal of the current source, and having the second impedance are provided.

TECHNICAL FIELD

The present invention relates to a current amplifier.

BACKGROUND ART

In an amplifier used in, for example, a transceiver for moving object, high linearity is required.

In Nonpatent Literature 1 mentioned later, a current amplifier including a first differential pair having two transistors, and a second differential pair having two transistors is disclosed.

This current amplifier includes a first current source connected between a source terminal of each of the two transistors that form the first differential pair, and ground, and a second current source connected between a source terminal of each of the two transistors that form the second differential pair, and the ground. The gain of this current amplifier is proportional to the ratio of a current outputted from the first current source and a current outputted from the second current source.

Further, in this current amplifier, a feedback resister is connected between the gate terminal and the drain terminal in each of the two transistors that form the first differential pair.

In this current amplifier, because the feedback resisters are connected, distortion in the dynamic range of a gate-to-source voltage in the two transistors that form the first differential pair is reduced. Namely, the distortion in the dynamic range of the gate-to-source voltage, the distortion being caused by the first differential pair, is reduced.

Because, as a result of the reduction in the distortion in the dynamic range, substantially the same voltage is applied to the first differential pair and the second differential pair, so that the linearity of the current amplifier is improved.

CITATION LIST Nonpatent Literature

Non Patent Literature 1: G. S. Sahota and C. J. Persico, “High dynamic range variable-gain amplifier for CDMA wireless applications,” in Digest of Technical Papers—IEEE International Solid-State Circuits Conference, 1997, vol. 40, pp. 374-375.

SUMMARY OF INVENTION Technical Problem

In the conventional current amplifier, because the second current source is connected between the source terminal of each of the two transistors that form the second differential pair, and the ground, a voltage occurs between both terminals of the second current source. When a voltage occurs between both the terminals of the second current source, due to the influence of that voltage, the peak amplitude of a voltage at each of the drain terminals of the two transistors that form the second differential pair becomes small. When the peak amplitude of the voltage at each of the drain terminals becomes small, the linearity degrades, and the linearity further degrades with increase in the decrease amount of the peak amplitude.

Namely, the conventional current amplifier has a problem that even though the distortion in the dynamic range of the gate-to-source voltage in the two transistors that form the first differential pair is reduced by using the above-mentioned feedback resister, the linearity still degrades.

The present invention is made in order to solve the above-mentioned problem, and it is therefore an object of the present invention to provide a current amplifier that can eliminate the second current source connected between the source terminal of each of the two transistors that forma second differential pair, and ground, thereby preventing degradation in linearity.

Solution to Problem

A current amplifier according to the present invention includes: a first transistor whose source terminal is grounded and whose gate terminal is configured to receive a first signal; a second transistor whose source terminal is grounded and whose gate terminal is configured to receive a second signal, the first signal and the second signal mutually constituting a differential signal; a third transistor whose source terminal is grounded, whose drain terminal is connected to a first output terminal, and whose gate terminal is configured to receive the first signal; a fourth transistor whose source terminal is grounded, whose drain terminal is connected to a second output terminal, and whose gate terminal is configured to receive the second signal; a first feedback circuit connected between the gate terminal and a drain terminal which are included in the first transistor, and having first impedance; a second feedback circuit connected between the gate terminal and a drain terminal which are included in the second transistor, and having the first impedance; a current source outputting a current; a first load circuit connected between the drain terminal of the first transistor and an output terminal of the current source, and having second impedance; and a second load circuit connected between the drain terminal of the second transistor and the output terminal of the current source, and having the second impedance. A differential inductor having a center tap terminal is used as the first load circuit and the second load circuit. The center tap terminal of the differential inductor is connected to the output terminal of the current source, a first input/output terminal of the differential inductor is connected to the drain terminal of the first transistor, and a second input/output terminal of the differential inductor is connected to the drain terminal of the second transistor.

Advantageous Effects of Invention

According to the present invention, because the configuration as described above is provided, the second current source connected between the source terminal of each of the two transistors that form a second differential pair and ground is not required. As a result, there is provided an effect of preventing degradation in linearity.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing a current amplifier according to Embodiment 1 of the present invention;

FIG. 2 is an explanatory drawing showing an example in which an input signal source 8 is connected between an input terminal 1 a and an input terminal 1 b in the current amplifier of FIG. 1;

FIG. 3 is a schematic diagram showing a current amplifier according to Embodiment 2 of the present invention;

FIG. 4 is a schematic diagram showing a current amplifier according to Embodiment 3 of the present invention;

FIG. 5 is an explanatory drawing showing a frequency response of the current amplifier;

FIG. 6 is a schematic diagram showing a current amplifier according to Embodiment 4 of the present invention;

FIG. 7 is an explanatory drawing showing a frequency response of a current amplifier;

FIG. 8 is a schematic diagram showing a current amplifier according to Embodiment 6 of the present invention;

FIG. 9 is an explanatory drawing showing a relation between a voltage amplitude V_(IN) of an input signal in an amplitude detector 23, and a current I₁,

FIG. 10 illustrates FIG. 2 including a load circuit that is a differential inductor;

FIG. 11 illustrates FIG. 4 including a load circuit that is a differential inductor;

FIG. 12 illustrates FIG. 6 including a load circuit that is a differential inductor; and

FIG. 13 illustrates FIG. 8 including a load circuit that is a differential inductor.

DESCRIPTION OF EMBODIMENTS

Hereafter, for explaining the present invention in greater detail, some embodiments of the present invention will be described with reference to the accompanying drawings.

Embodiment 1

FIG. 1 is a schematic diagram showing a current amplifier according to Embodiment 1 of the present invention.

In FIG. 1, an input terminal 1 a is a terminal to which a first signal is inputted.

An input terminal 1 b is a terminal to which a second signal forming a differential signal with the first signal is inputted.

In this Embodiment 1, an electric potential difference between the first signal inputted from the input terminal 1 a and the second signal inputted from the input terminal 1 b is referred to as an input voltage IN.

A first differential pair 2 includes a first transistor 2 a and a second transistor 2 b.

The first transistor 2 a and the second transistor 2 b are, for example, field effect transistors such as MOS (Metal Oxide Semiconductor) transistors.

A source terminal of the first transistor 2 a is grounded, and a gate terminal of the first transistor 2 a is connected to the input terminal 1 a. The first signal is provided to the gate terminal of the first transistor 2 a.

A source terminal of the second transistor 2 b is grounded, and a gate terminal of the first transistor 2 b is connected to the input terminal 1 b. The second signal is provided to the gate terminal of the second transistor 2 b.

A second differential pair 3 includes a third transistor 3 a and a fourth transistor 3 b.

The third transistors 3 a and the fourth transistor 3 b are, for example, field effect transistors such as MOS transistors.

A source terminal of the third transistor 3 a is grounded, a gate terminal of the third transistor 3 a is connected to the input terminal 1 a, and a drain terminal of the third transistor 3 a is connected to a first output terminal 7 a. The first signal is provided to the gate terminal of the third transistor 3 a.

A source terminal of the fourth transistor 3 b is grounded, a gate terminal of the fourth transistor 3 b is connected to the input terminal 1 b, and a drain terminal of the fourth transistor 3 b is connected to a second output terminal 7 b. The second signal is provided to the gate terminal of the fourth transistor 3 b.

A first feedback circuit 4 a is connected between the gate terminal and a drain terminal in the first transistor 2 a, and has first impedance Z₁.

A second feedback circuit 4 b is connected between the gate terminal and a drain terminal in the second transistor 2 b. The second feedback circuit 4 b has the first impedance Z₁ as impedance being the same characteristics as the first impedance Z₁ that the first feedback circuit 4 a has.

Although it is assumed that the first feedback circuit 4 a and the second feedback circuit 4 b are implemented by, for example, resistors, as long as the feedback circuits have the first impedance Z₁, it does not matter what circuit configuration the feedback circuits have.

A current source 5 has an input terminal and an output terminal. The input terminal of the current source 5 is connected to a power source line having a voltage V_(dd).

A first load circuit 6 a is connected between the drain terminal of the first transistor 2 a and the output terminal of the current source 5, and has second impedance Z₂.

A second load circuit 6 b is connected between the drain terminal of the second transistor 2 b and the output terminal of the current source 5. The second load circuit 6 b has the second impedance Z₂ as impedance being the same characteristics as the second impedance Z₂ that the first load circuit 6 a has.

Although it is assumed that the first load circuit 6 a and the second load circuit 6 b are implemented by, for example, resistors, as long as the load circuits have the second impedance Z₂, it does not matter what circuit configuration the load circuits have.

The first output terminal 7 a is a terminal from which the first signal amplified by the first transistor 2 a and the third transistor 3 a is outputted.

The second output terminal 7 b is a terminal from which the second signal amplified by the second transistor 2 b and the fourth transistor 3 b is outputted.

A capacitor 10 a is a capacitive component that blocks a DC component included in the first signal inputted from the input terminal 1 a.

A capacitor 10 b is a capacitive component that blocks a DC component included in the second signal inputted from the input terminal 1 b.

In this Embodiment 1, a current outputted from the first output terminal 7 a and a current outputted from the second output terminal 7 b are referred to as output currents I_(OUT).

Next, operations will be explained.

A bias is applied between the gate and the source of each of the first transistor 2 a and the second transistor 2 b that form the first differential pair 2 by the current outputted from the current source 5.

The first signal inputted from the input terminal 1 a and whose DC component is blocked by the capacitor 10 a is provided to the gate terminal of the first transistor 2 a. The second signal inputted from the input terminal 1 b and whose DC component is blocked by the capacitor 10 b is provided to the gate terminal of the second transistor 2 b.

Thus, at the drain terminal of the second transistor 2 a, a voltage amplitude corresponding to the first signal appears due to the second impedance Z₂ that the first load circuit 6 a has.

Further, at the drain terminal of the second transistor 2 b, a voltage amplitude corresponding to the second signal appears due to the second impedance Z₂ that the second load circuit 6 b has.

The voltage amplitude corresponding to the first signal is fed back to the gate terminal of the first transistor 2 a by the first feedback circuit 4 a.

Further, the voltage amplitude corresponding to the second signal is fed back to the gate terminal of the second transistor 2 b by the second feedback circuit 4 b.

The linearity of the drain current of the first transistor 2 a with respect to the first signal inputted from the input terminal 1 a is improved through the feedback action of the voltage amplitude corresponding to the first signal.

Further, the linearity of the drain current of the second transistor 2 b with respect to the second signal inputted from the input terminal 1 b is improved through the feedback action of the voltage amplitude corresponding to the second signal.

The drain current of the third transistor 3 a that forms the second differential pair 3 is amplified in proportion to the transistor size ratio of the third transistor 3 a and the first transistor 2 a.

Further, the drain current of the fourth transistor 3 b that forms the second differential pair 3 is amplified in proportion to the transistor size ratio of the fourth transistor 3 b and the second transistor 2 b.

Conventionally, there exists a current amplifier in which the first differential pair 2 is not disposed and a second differential pair 3 is connected directly to the input terminals 1 a and 1 b. In the current amplifier of this Embodiment 1, the linearity of the output currents I_(OUT) with respect to the input voltage IN from the input terminals 1 a and 1 b is improved in comparison with that of the conventional current amplifier.

FIG. 2 is an explanatory drawing showing an example in which an input signal source 8 is connected between the input terminal 1 a and the input terminal 1 b shown in the current amplifier of FIG. 1.

In FIG. 2, V_(IN) denotes the voltage amplitude of the input signal source 8 connected between the input terminal 1 a and the input terminal 1 b, and R_(s) denotes the output impedance of the input signal source 8. I_(OUT) denotes the current outputted from the first output terminal 7 a and the second output terminal 7 b.

When the transconductance of each of the first and second transistors 2 a and 2 b is denoted by g_(m1), the transconductance G_(m) in the current amplifier of FIG. 2 is expressed as shown in the following equation (1).

$\begin{matrix} {G_{m} = {\frac{I_{OUT}}{V_{IN}} = {\frac{- 1}{R_{S}} \times \frac{\frac{R_{S}Z_{1}}{R_{S} + Z_{1}} \times g_{m\; 1}}{1 + \left( {\frac{Z_{1}Z_{2}}{Z_{1} + Z_{2}} \times g_{m\; 1} \times \frac{R_{S}}{R_{S} + Z_{1}}} \right)} \times \frac{I_{2}}{I_{1}}}}} & (1) \end{matrix}$

In equation (1), I₁ denotes a current flowing between the drain and the source in each of the first and second transistors 2 a and 2 b, and I₂ denotes a current flowing between the drain and the source in each of the third and fourth transistors 3 a and 3 b.

In equation (1), the transconductance g_(m1) in each of the first and second transistors 2 a and 2 b exhibits nonlinearity.

However, as shown in the following equation (2), when Z₁, Z₂, g_(m1) and R_(s) are set in such a way that the second term of the denominator in equation (1) becomes sufficiently larger than 1, g_(m1) in the numerator of equation (1) and g_(m1) in the denominator of equation (1) can be canceled approximately. As a result, the transconductance G_(m) exhibits linearity.

$\begin{matrix} {{{Second}\mspace{14mu}{Term}\mspace{14mu}{of}\mspace{14mu}{Denominator}\mspace{14mu}{in}\mspace{14mu}{Equation}\mspace{14mu}(1)} = {\left( {\frac{Z_{1}Z_{2}}{Z_{1} + Z_{2}} \times g_{m\; 1} \times \frac{R_{S}}{R_{S} + Z_{1}}} \right) ⪢ 1}} & (2) \end{matrix}$

As is clear from the above description, according to this Embodiment 1, the following components are included: a first transistor whose source terminal is grounded and whose gate terminal is configured to receive a first signal; a second transistor whose source terminal is grounded and whose gate terminal is configured to receive a second signal, the first signal and the second signal mutually constituting a differential signal; a third transistor whose source terminal is grounded, whose drain terminal is connected to a first output terminal, and whose gate terminal is configured to receive the first signal; a fourth transistor whose source terminal is grounded, whose drain terminal is connected to a second output terminal, and whose gate terminal is configured to receive the second signal; a first feedback circuit 4 a connected between the gate terminal and a drain terminal which are included in the first transistor 2 a, and having first impedance Z₁; a second feedback circuit 4 b connected between the gate terminal and a drain terminal which are included in the second transistor 2 b, and having the first impedance Z₁; a current source 5 outputting a current; a first load circuit 6 a connected between the drain terminal of the first transistor 2 a and an output terminal of the current source 5, and having second impedance Z₂; and a second load circuit 6 b connected between the drain terminal of the second transistor 2 b and the output terminal of the current source 5, and having the second impedance Z₂. As a result, a second current source connected between the ground and each of the source terminal of the third transistor 3 a and the source terminal of the fourth transistor 3 b is not required, and there is provided an effect of being able to avoid degradation in the linearity.

Namely, according to this Embodiment 1, in each of the third and fourth transistors 3 a and 3 b, the source terminal is grounded, and no current source is connected between the source terminal and the ground. Therefore, in the current amplifier of this Embodiment 1, the peak amplitude of the voltage of the drain terminal in each of the third and fourth transistors 3 a and 3 b does not become small due to the influence of a current source, and degradation in the linearity does not occur, unlike in the case of the current amplifier described in Nonpatent Literature 1. Therefore, the linearity can be improved comparing with than that of the current amplifier described in Nonpatent Literature 1.

Embodiment 2

In this Embodiment 2, a current amplifier that includes, as first and second load circuits, a differential inductor 9 having a center tap terminal 9 a will be explained.

FIGS. 10-13 illustrate the schematic diagrams if FIGS. 2, 4, 6 and 8 that include a differential inductor 9 as discussed above.

FIG. 3 is a schematic diagram showing the current amplifier according to Embodiment 2 of the present invention. In FIG. 3, because the same reference characters as those shown in FIG. 1 denote the same or corresponding components, an explanation of the components will be omitted hereafter.

The differential inductor 9 has the center tap terminal 9 a.

The center tap terminal 9 a of the differential inductor 9 is connected to an output terminal of a current source 5, a first input/output terminal 9 b of the differential inductor 9 is connected to a drain terminal of a first transistor 2 a, and a second input/output terminal 9 c of the differential inductor 9 is connected to a drain terminal of a second transistor 2 b.

A part of the differential inductor 9 extending from the center tap terminal 9 a to the first input/output terminal 9 b has second impedance Z₂, like the first load circuit 6 a of FIG. 1.

Further, a part of the differential inductor 9 extending from the center tap terminal 9 a to the second input/output terminal 9 c has the second impedance Z₂, like the second load circuit 6 b of FIG. 1.

Due to such a configuration, the differential inductor 9 operates in the same way as the first load circuit 6 a and the second load circuit 6 b of FIG. 1.

In above-described Embodiment 1, in a case in which noise signals are inputted as in-phase signals from the input terminals 1 a and 1 b, a voltage corresponding to the in-phase signals occurs at a terminal in the first load circuit 6 a, the terminal being on a side of the first transistor 2 a. Further, a voltage corresponding to the in-phase signals occurs at a terminal in the second load circuit 6 b, the terminal being on a side of the second transistor 2 b. Therefore, amplified in-phase signals are outputted from the first output terminal 7 a and the second output terminal 7 b.

In this Embodiment 2, even in a case in which noise signals are inputted as in-phase signals from the input terminals 1 a and 1 b, a voltage corresponding to the in-phase signals does not occur at the first input/output terminal 9 b of the differential inductor 9. Further, in the same case, a voltage corresponding to the in-phase signals does not occur at the second input/output terminal 9 c of the differential inductor 9. Therefore, the amplified in-phase signals can be prevented from being outputted from the first output terminal 7 a and the second output terminal 7 b.

As is clear from the above description, according to this Embodiment 2, a differential inductor having a center tap terminal is used as the first load circuit and the second load circuit, the center tap terminal of the differential inductor is connected to the output terminal of the current source, a first input/output terminal of the differential inductor is connected to the drain terminal of the first transistor, and a second input/output terminal of the differential inductor is connected to the drain terminal of the second transistor. Therefore, there is provided an effect of being able to increase a common-mode rejection ratio (CMRR) comparing with that in above-described Embodiment 1, in addition to the same effects as those provided by above-described Embodiment 1.

Embodiment 3

In this Embodiment 3, an example in which two current amplifiers each has the configuration shown in FIG. 1 or 3 are connected in parallel will be explained.

FIG. 4 is a schematic diagram showing a current amplifier according to Embodiment 3 of the present invention. In FIG. 4, the same reference characters as those shown in FIG. 1 denote the same or corresponding components.

Although an example in which two current amplifiers each has a configuration shown in FIG. 1 are connected in parallel is shown in FIG. 4, two current amplifiers each has a configuration shown in FIG. 3 may be connected in parallel.

Each of the first and second amplifier circuits 11 and 12 corresponds to the current amplifier shown in FIG. 1.

A first feedback circuit 4 a and a second feedback circuit 4 b in the first amplifier circuit 11 have the same role as the first feedback circuit 4 a and the second feedback circuit 4 b in the current amplifier of FIG. 1. To this end, the first feedback circuit 4 a and the second feedback circuit 4 b in the first amplifier circuit 11 have first impedance as impedance being the same characteristics, like the first feedback circuit 4 a and the second feedback circuit 4 b in the current amplifier of FIG. 1.

Further, a first feedback circuit 4 a and a second feedback circuit 4 b in the second amplifier circuit 12 have the same role as the first feedback circuit 4 a and the second feedback circuit 4 b in the current amplifier of FIG. 1. To this end, the first feedback circuit 4 a and the second feedback circuit 4 b in the second amplifier circuit 12 have first impedance as impedance being the same characteristics, like the first feedback circuit 4 a and the second feedback circuit 4 b in the current amplifier of FIG. 1.

However, because the first amplifier circuit 11 and the second amplifier circuit 12 have different characteristics, there is a difference between the first impedance that the first feedback circuit 4 a and the second feedback circuit 4 b in the first amplifier circuit 11 have, and the first impedance that the first feedback circuit 4 a and the second feedback circuit 4 b in the second amplifier circuit 12 have.

In the example shown in FIG. 4, the first impedance that the first feedback circuit 4 a and the second feedback circuit 4 b in the first amplifier circuit 11 have is Z_(1a), and the first impedance that the first feedback circuit 4 a and the second feedback circuit 4 b in the second amplifier circuit 12 have is Z_(1b).

A first load circuit 6 a and a second load circuit 6 b in the first amplifier circuit 11 have the same role as the first load circuit 6 a and the second load circuit 6 b in the current amplifier of FIG. 1. To this end, the first load circuit 6 a and the second load circuit 6 b in the first amplifier circuit 11 have second impedance as impedance being the same characteristics, like the first load circuit 6 a and the second load circuit 6 b in the current amplifier of FIG. 1.

Further, a first load circuit 6 a and a second load circuit 6 b in the second amplifier circuit 12 have the same role as the first load circuit 6 a and the second load circuit 6 b in the current amplifier of FIG. 1. To this end, the first load circuit 6 a and the second load circuit 6 b in the second amplifier circuit have second impedance as impedance being the same characteristics, like the first load circuit 6 a and the second load circuit 6 b in the current amplifier of FIG. 1.

However, because the first amplifier circuit 11 and the second amplifier circuit 12 have different characteristics, there is a difference between the second impedance that the first load circuit 6 a and the second load circuit 6 b in the first amplifier circuit 11 have, and the second impedance that the first load circuit 6 a and the second load circuit 6 b in the second amplifier circuit 12 have.

In the example shown in FIG. 4, the second impedance that the first load circuit 6 a and the second load circuit 6 b in the first amplifier circuit 11 is Z_(2a), and the second impedance that the first load circuit 6 a and the second load circuit 6 b in the second amplifier circuit 12 have is Z_(2b).

Next, operations will be explained.

A first signal is inputted from an input terminal 1 a in the first amplifier circuit 11, and the same signal as the first signal is inputted from an input terminal 1 a in the second amplifier circuit 12.

Further, a second signal is inputted from an input terminal 1 b in the first amplifier circuit 11, and the same signal as the second signal is inputted from an input terminal 1 b in the second amplifier circuit 12.

Thus, the first signal amplified by the first amplifier circuit 11 and the first signal amplified by the second amplifier circuit 12 are synthesized, and the amplified first signal after synthesis is outputted from a first output terminal 7 a.

Further, the second signal amplified by the first amplifier circuit 11 and the second signal amplified by the second amplifier circuit 12 are synthesized, and the amplified second signal after synthesis is outputted from a second output terminal 7 b.

Here, FIG. 5 is an explanatory drawing showing a frequency response of the current amplifier.

In FIG. 5, a relation between the frequency and the transconductance G_(m) in the current amplifier is shown as the frequency response of the current amplifier.

In FIG. 5, a broken line shows a case of the current amplifier of Embodiment 1, and a solid line shows a case of the current amplifier of Embodiment 3.

In this Embodiment 3, there is a difference between the first impedance that the first feedback circuit 4 a and the second feedback circuit 4 b in the first amplifier circuit 11 have, and the first impedance that the first feedback circuit 4 a and the second feedback circuit 4 b in the second amplifier circuit 12 have. Further, there is a difference between the second impedance that the first load circuit 6 a and the second load circuit 6 b in the first amplifier circuit 11 have, and the second impedance that the first load circuit 6 a and the second load circuit 6 b in the second amplifier circuit 12 have.

Therefore, there is a difference between the transconductance G_(m) corresponding to the frequency in the first amplifier circuit 11, and the transconductance G_(m) corresponding to the frequency in the second amplifier circuit 12.

Therefore, in the case of the current amplifier of this Embodiment 3, as shown in FIG. 5, a flat frequency response is acquired within a wider band than that in a case in which only one current amplifier is disposed like in the case of above-described Embodiment 1.

Embodiment 4

In above-described Embodiment 3, the example in which each of the first and second amplifier circuits 11 and 12 includes a current source 5 outputting a fixed output current is shown.

In this Embodiment 4, an example in which each of first and second amplifier circuits 11 and 12 includes a current source 21 outputting a variable current and a control circuit 22 controlling the current outputted from the current source 21 will be explained.

FIG. 6 is a schematic diagram showing a current amplifier according to Embodiment 4 of the present invention. In FIG. 6, the same reference characters as those shown in FIG. 4 denote the same or corresponding components.

Both the current source 21 included in the first amplifier circuit 11 and the current source 21 included in the second amplifier circuit 12 are variable current sources each having an input terminal and an output terminal. An input terminal of the current source 21 included in the first amplifier circuit 11 is connected to a power source line having a voltage V_(dd) in the first amplifier circuit 11, and an output terminal of the current source 21 in the first amplifier circuit 11 is connected to both an end of a first load circuit 6 a in the first amplifier circuit 11 and an end of a second load circuit 6 b in the first amplifier circuit 11. An input terminal of the current source 21 included in the second amplifier circuit 12 is connected to a power source line having a voltage V_(dd) in the first amplifier circuit 12, and an output terminal of the current source 21 in the second amplifier circuit 12 is connected to both an end of a first load circuit 6 a in the second amplifier circuit 12 and an end of a second load circuit 6 b in the second amplifier circuit 12.

The control circuit 22 controls the current outputted from the current source 21 included in the first amplifier circuit 11, and also controls the current outputted from the current source 21 included in the second amplifier circuit 12.

FIG. 7 is an explanatory drawing showing a frequency response of the current amplifier.

In FIG. 7, a relation between the frequency and the transconductance G_(m) in the current amplifier is shown as the frequency response of the current amplifier.

Next, operations will be explained.

When the control circuit 22 performs control in such a way that the current outputted from the current source 21 included in the first amplifier circuit 11 and the current outputted from the current source 21 included in the second amplifier circuit 12 have the same value, the frequency response of the current amplifier represents a curve as shown by A in FIG. 7.

When the control circuit 22 performs control in such a way that the current outputted from the current source 21 included in the first amplifier circuit 11 is greater than the current outputted from the current source 21 s included in the second amplifier circuit 12, the frequency response of the current amplifier represents a curve as shown by B in FIG. 7.

When the control circuit 22 performs control in such a way that the current outputted from the current source 21 included in the first amplifier circuit 11 is less than the current outputted from the current source 21 included in the second amplifier circuit 12, the frequency response of the current amplifier becomes a curve as shown by C in FIG. 7.

Therefore, by controlling the current outputted from the current source 21 included in the first amplifier circuit 11 and the current outputted from the current source 21 included in the second amplifier circuit 12 by means of the control circuit 22, the frequency response of the current amplifier can be adjusted.

Embodiment 5

Although in above-described Embodiments 1 to 4, an example in which the first transistor 2 a, the second transistor 2 b, the third transistor 3 a, and the fourth transistor 3 b are field effect transistors is shown, the first through fourth transistors are not limited to field effect transistors.

For example, the first transistor 2 a, the second transistor 2 b, the third transistor 3 a, and the fourth transistor 3 b may be bipolar transistors.

In the case in which bipolar transistors are used as the first transistor 2 a, the second transistor 2 b, the third transistor 3 a, and the fourth transistor 3 b, the base terminals of the bipolar transistors correspond to the gate terminals of the first through fourth transistors shown in above-described Embodiments 1 to 4.

Further, the emitter terminals of the bipolar transistors correspond to the source terminals of the first through fourth transistors shown in above-described Embodiments 1 to 4, and the collector terminals of the bipolar transistors correspond to the drain terminals of the first through fourth transistors shown in above-described Embodiments 1 to 4.

Also in the case in which bipolar transistors are used as the first through fourth transistors, the same effects as those provided by above-described Embodiments 1 to 4 can be achieved.

Embodiment 6

In above-described Embodiments 1 to 5, the example in which the current amplifier includes a current source 5 is shown.

In this Embodiment 6, an example in which an amplitude detector 23 is included as a current source 5 will be explained.

FIG. 8 is a schematic diagram showing a current amplifier according to Embodiment 6 of the present invention. In FIG. 8, the same reference characters as those shown in FIG. 1 denote the same or corresponding components, so that an explanation of the components will be omitted hereafter.

The amplitude detector 23 is a current source that includes a square-law detector circuit 24, constant current sources 25 and 26, and a current mirror circuit 27, and is a circuit that outputs a current (I₀+I_(d)) that is the sum of a current directly proportional to the square value of the voltage amplitude of an inputted signal, and a fixed current.

The square-law detector circuit 24 includes a transistor 24 a and a transistor 24 b, and outputs a current I_(b) directly proportional to the square value of the voltage amplitude V_(IN) of the inputted signal.

Each of the transistors 24 a and 24 b is a field effect transistor such as a MOS.

A source terminal of the transistor 24 a is grounded, and a signal having the voltage amplitude V_(IN) is provided to a gate terminal of the transistor 24 a.

A source terminal of the transistor 24 b is grounded, a drain terminal of the transistor 24 b is connected to a drain terminal of the transistor 24 a, and a signal having the voltage amplitude V_(IN) is provided to a gate terminal of the transistor 24 a.

The constant current source 25 outputs a current I_(a) that is a fixed current.

The constant current source 25 has an input terminal and an output terminal, the input terminal of the constant current source 25 is connected to a power source line V_(dd), and the output terminal of the constant current source 25 is connected to the drain terminal of the transistor 24 a and the drain terminal of the transistor 24 b.

The constant current source 26 outputs a current I_(d) that is a fixed current.

The constant current source 26 has an input terminal and an output terminal, the input terminal of the constant current source 26 is connected to the power source line V_(dd), and the output terminal of the constant current source 26 is connected to a drain terminal of a P-channel MOS transistor 27 b in the current mirror circuit 27, a first load circuit 6 a, and a second load circuit 6 b.

The current mirror circuit 27 includes a P-channel MOS transistor 27 a and the P-channel MOS transistor 27 b, and outputs a current I₀ on the basis of the current I_(b) outputted from the square-law detector circuit 24 and the current I_(a) outputted from the constant current source 25.

In the P-channel MOS transistor 27 a, a source terminal is connected to the power source line V_(dd), and a drain terminal and a gate terminal are connected to the output terminal of the constant current source 25, the drain terminal of the transistor 24 a, and the drain terminal of the transistor 24 b.

In the P-channel MOS transistor 27 b, a gate terminal is connected to the gate terminal of the P-channel MOS transistor 27 a, a source terminal is connected to the power source line V_(dd), and the drain terminal is connected to the output terminal of the constant current source 26, the first load circuit 6 a, and the second load circuit 6 b.

A resistor 28 a has an end connected to a power source line V_(dc), and another end connected to the gate terminal of the transistor 24 a.

A resistor 28 b has an end connected to the power source line V_(dc), and another end connected to the gate terminal of the transistor 24 b.

A capacitor 29 a has an end connected to an input terminal 1 a, and another end connected to the gate terminal of the transistor 24 a.

A capacitor 29 b has an end connected to an input terminal 1 b, and another end connected to the gate terminal of the transistor 24 b.

Next, operations will be explained.

Because components other than the amplitude detector 23 are the same as those of above-described Embodiment 1, only an operation of the amplitude detector 23 will be explained hereafter.

Because an end of the resistor 28 a is connected to the power source line V_(dc), a signal having a voltage amplitude V_(IN) is provided to the gate terminal of the transistor 24 a of the square-law detector circuit 24.

Further, because an end of the resistor 28 b is connected to the power source line V_(dc), a signal having the voltage amplitude V_(IN) is provided to the gate terminal of the transistor 24 b of the square-law detector circuit 24.

The voltage amplitude V_(IN) is a voltage that is a result obtained by subtracting a voltage drop in the resistor 28 a or 28 b from the voltage of the power source line V_(dc).

When a signal having the voltage amplitude V_(IN) is provided to the gate terminal of each of the transistors 24 a and 24 a, the square-law detector circuit 24 outputs a current I_(b) directly proportional to the square value of the voltage amplitude V_(IN) of the inputted signal to the current mirror circuit 27.

The current I_(b) is represented by the following equation (3). I _(b) =I _(c) +K _(i) V _(IN) ²  (3)

In equation (3), I_(c) denotes a constant direct current included in the current I_(b), and K_(i) denotes a constant of proportionality.

The current mirror circuit 27 outputs a current I₀ as represented in the following equation (4) to the first load circuit 6 a and the second load circuit 6 b on the basis of the current I_(b) outputted from the square-law detector circuit 24 and the current I_(a) outputted from the constant current source 25. I ₀ =K ₂((I _(c) −I _(a))+K ₁ V _(IN) ²)  (4)

In equation (4), K₂ denotes a mirror ratio of the current mirror circuit 27.

Then, a current that is the sum of the current I₀ outputted from the current mirror circuit 27 and the current I_(d) outputted from the constant current source 25 is outputted to the first load circuit 6 a and the second load circuit 6 b.

As a result, a current I₁ flowing between a drain and a source in each of first and second transistors 2 a and 2 b is represented by the following equation (5). I ₁ =I ₀ +I _(d) =K ₂(I _(C) −I _(a))+I _(d) +K ₁ K ₂ V _(IN) ²  (5)

Hereafter, some effects of this Embodiment 6 will be explained.

In a region in which the voltage amplitude V_(IN) of the input signal is small, the current I₀ outputted from the amplitude detector 23 is small, and the current I₁ flowing between the drain and the source in each of the first and second transistors 2 a and 2 b is also small.

For example, signals recently used for mobile communications and so on are ones on which multi-level modulation is performed, and there is a tendency that their peak to average power ratios are large. In a case in which the current amplifier is used for such applications, the average current consumption can be reduced.

By adjusting the current I_(a), the current I_(d), and the mirror ratio K₂, the inclination of the current I₁ with respect to the voltage amplitude V_(IN) of the input signal can be changed, as shown in FIG. 9.

FIG. 9 is an explanatory drawing showing relations between the voltage amplitude V_(IN) of the input signal and the current I₁ in the amplitude detector 23.

In an example of FIG. 9, characteristics expressed by a solid line and characteristics expressed by a broken line are represented as two characteristics that the inclination of the current I₁ with respect to the voltage amplitude V_(IN) of the input signal differs.

It is to be understood that any combination of two or more of the above-described embodiments can be made, various changes can be made in any component according to any one of the above-described embodiments, and any component according to any one of the above-described embodiments can be omitted within the scope of the present invention.

INDUSTRIAL APPLICABILITY

The present invention is suitable for a current amplifier having multiple transistors.

REFERENCE SIGNS LIST

-   1 a, 1 b input terminal, 2 first differential pair, 2 a first     transistor, 2 b second transistor, 3 second differential pair, 3 a     third transistor, 3 b fourth transistor, 4 a first feedback circuit,     4 b second feedback circuit, 5 current source, 6 a first load     circuit, 6 b second load circuit, 7 a first output terminal, 7 b     second output terminal, 8 input signal source, 9 differential     inductor, 9 a center tap terminal, 9 b first input/output terminal,     9 c second input/output terminal, 10 a, 10 b capacitor, 11 first     amplifier circuit, 12 second amplifier circuit, 21 current source,     22 control circuit, 23 amplitude detector, 24 square-law detector     circuit, 24 a, 24 b transistor, 25, 26 constant current source, 27     current mirror circuit, 27 a, 27 b P-channel MOS transistor, 28 a,     28 b resistor, and 29 a, 29 b capacitor. 

The invention claimed is:
 1. A current amplifier comprising: a first transistor whose source terminal is grounded and whose gate terminal is configured to receive a first signal; a second transistor whose source terminal is grounded and whose gate terminal is configured to receive a second signal, the first signal and the second signal mutually constituting a differential signal; a third transistor whose source terminal is grounded, whose drain terminal is connected to a first output terminal, and whose gate terminal is configured to receive the first signal; a fourth transistor whose source terminal is grounded, whose drain terminal is connected to a second output terminal, and whose gate terminal is configured to receive the second signal; a first feedback circuit connected between the gate terminal and a drain terminal which are included in the first transistor, and having first impedance; a second feedback circuit connected between the gate terminal and a drain terminal which are included in the second transistor, and having the first impedance; a current source outputting a current; a first load circuit connected between the drain terminal of the first transistor and an output terminal of the current source, and having second impedance; and a second load circuit connected between the drain terminal of the second transistor and the output terminal of the current source, and having the second impedance, wherein a differential inductor having a center tap terminal is used as the first load circuit and the second load circuit, the center tap terminal of the differential inductor is connected to the output terminal of the current source, a first input/output terminal of the differential inductor is connected to the drain terminal of the first transistor, and a second input/output terminal of the differential inductor is connected to the drain terminal of the second transistor.
 2. The current amplifier according to claim 1, wherein a first amplifier circuit including the first through the fourth transistors, the first and second feedback circuits, the current source, and the first and second load circuits, and a second amplifier circuit including the first through fourth transistors, the first and second feedback circuits, the current source, and the first and second load circuits that are different from those included in the first amplifier circuit, are connected in parallel, and the first impedance that the first and second feedback circuits included in the first amplifier circuit have differs from the first impedance that the first and second feedback circuits included in the second amplifier circuit have, and the second impedance that the first and second load circuits included in the first amplifier circuit have differs from the second impedance that the first and second load circuits included in the second amplifier circuit have.
 3. The current amplifier according to claim/further comprising a control circuit controlling both the current outputted from the current source included in the first amplifier circuit and the current outputted from the current source included in the second amplifier circuit.
 4. The current amplifier according to claim 1, wherein each of the first through fourth transistors is a field effect transistor.
 5. The current amplifier according to claim 1, wherein each of the first through fourth transistors is a bipolar transistor, a base terminal of the bipolar transistor is the gate terminal, an emitter terminal of the bipolar transistor is the source terminal, and a collector terminal of the bipolar transistor is the drain terminal.
 6. The current amplifier according to claim 1, wherein the current source outputs the current that is a sum of a current directly proportional to a square value of a voltage amplitude of an inputted signal, and a fixed current. 